This invention relates generally to electronic touch screens and, more particularly, to methods and systems for securing data entered through a touch screen.
Electronic touch screens that provide coordinate data regarding the location of an object being brought into proximity to a screen are well known. Typically, keypads are displayed on touch screens to receive user input for application programs. These keypads are comprised of a plurality of keys that are displayed on a screen. Each keypad typically is defined by two or more corner coordinates and/or length and width parameters. These keypad data define areas on the screen that correspond to particular keys. In response to an object being brought into proximity to the screen, the screen generates location coordinates for the xe2x80x98touchxe2x80x99 and a screen control program determines whether the coordinates of the xe2x80x98touchxe2x80x99 correspond to one of the defined keypad areas. If they do, the screen control program retrieves input data that correspond to the keypad area that was xe2x80x98touchedxe2x80x99 and this input data are provided to an application program. Otherwise, no input data are recognized as being generated from the touch screen and exception processing may occur to indicate an erroneous touch to the user.
Typically, a touch screen generates coordinates for a location where an object is brought into proximity to the screen. The screen may be a resistive touch screen that is comprised of two planes of resistive material that are electrically insulated from one another and generally parallel to one another. To detect the location of a touch to the screen, a reference voltage is applied to one of the planes. This plane is called the xe2x80x98activexe2x80x99 plane. A location signal for a touch occurs when the force of the touch causes an electrical contact between the two planes and the voltage present at the other plane is measured. With linear resistance in the active plane, the location of the point of contact is directly proportional to the distance the contact point lies from the voltage source. This location gives the proportionate distance along the axis of the active plane. The voltage is then removed from the active plane and applied to the other plane. This action reverses the roles of the two planes so voltage measurement of the other plane provides a proportionate distance along the other axis.
Such touch screens may be used in applications where the data entered through the screen requires security. For example, a touch screen keypad may be used for the entry of alphanumeric data that are interrogated for determining whether access to a room or a financial account is allowed. One way an intruder may endeavor to obtain this data to gain unauthorized access to a location or account is to surreptitiously observe the entry of the data so the intruder may later use it. However, the authorized person may be cautious enough to detect any such effort to observe the data entry.
To avoid being noticed by an authorized person entering data, the intruder may couple a voltage measurement device to the touch screen planes and record the voltage changes on the planes. By observing the reference voltages and the changes in the reference voltages on the planes, the intruder may determine the locations of touches that an authorized user applied to the screen. The intruder may then be able to coordinate the touch locations with keys on the screen to determine the alphanumeric key sequence for access to a location or account.
What is needed is a method for frustrating an intruder""s ability to detect the locations of touches on a touch screen from voltages measured on the planes of the touch screen.
The above-noted limitations of previously known touch screens have been overcome by a system and method made in accordance with the principles of the present invention. The method of the present invention is comprised of applying a varying reference voltage to a first plane of a touch screen with a time of application duration being substantially less than a touch period and measuring a voltage present on a second plane during a portion of the application duration of the voltage to the first plane.
Observation of the voltage on the active plane provides a series of voltage measurements that vary in magnitude and that remain present only for the length of the application period. Because the first or active plane is coupled to the second plane for a time period that corresponds to the touch period and because the touch period is longer than the application period, the voltage present on the second plane during the application period probably remains at only one level. That level is the reference voltage applied during the application period multiplied by the ratio of the resistance of the active plane at the point of contact to the total resistance of the active plane along its axis. That is, the intruder may not measure a voltage transition from the reference voltage to the voltage indicative of the touch location because the load coupled by the touch to the reference voltage is present from the beginning to the end of the application period. Because the reference voltage is randomly varied, observation of voltages present on the first plane does not identify a constant reference voltage or a predictable pattern of reference voltages. In previously known methods where the reference voltage remained constant or varied in an observable pattern, the reference voltage could be determined. With that information and knowledge of the voltage on the second plane, which is the voltage corresponding to the touch, the distance of the touch from the voltage source could be determined. Thus, the method of the present invention frustrates an intruder""s ability to determine the location of a touch by measuring the voltage on the second plane because the intruder is unable to accurately determine the reference voltage.
In a system implementing the principles of the present invention, a randomly varying reference source is coupled to a first plane of a touch screen for an application period that is significantly less than the touch period. A voltage measurement device is coupled to a second plane of the touch screen to measure the voltage present on the second plane when a touch occurs that couples the first plane to the second plane. The ratio of the measured voltage to the reference voltage may be used to determine the location coordinate of a touch along the axis of the first plane. The controller then applies a reference voltage to the second plane and the measurement device measures the voltage present on the first plane. The ratio computed from these two voltages may then be used to determine the location coordinate along the axis of the second plane. The location coordinates for these two operations define the location of the touch. In another type of system known as a xe2x80x98five-wirexe2x80x99 system, the voltage potential is still applied to the first plane but in a direction that is orthogonal to the application of the voltage for determination of the first location coordinate. Again, measurement of the voltage on the second plane at the touch point is used to compute the second location coordinate for the touch point.
The system and method of the present invention apply a randomly varying voltage to a first plane of a touch screen during application periods that are significantly smaller than the time that a touch is applied to the touch screen. The random variation of the reference voltage frustrates an intruder from determining the reference voltage and the short application period substantially reduces the likelihood that a transition occurs during a touch. By reducing the likelihood of a transition, an intruder is probably unable to determine the reference voltage and the measured voltage during measurement of a voltage caused by a touch.
These and other advantages and features of the present invention may be discerned from reviewing the accompanying drawings and the detailed description of the invention.